1. Field of the Invention
The present invention relates to a method for manufacturing high strength rail of excellent weldability.
2. Description of Prior Art
Modern railroads have stringent requirements for rails, particularly in the areas of high speed transit as well as heavy load service. Problems in such areas include fractured running surfaces and abrasion at the sides of rail heads, thus creating a demand for high strength rails which are able to withstand severe service conditions.
Moreover, most rails in current production are of the long rail type, which rails are produced by welding standard size rails to each other. In these types of rails, it is desired to avoid fracture at the joint portions thereof as well as to decrease the labor cost for track maintenance. Therefore, good weldability and homogeneity in the quality of the rail at the weld are also required for said high strength rails.
However, techniques for production of high strength rails to date have not taken into account deterioration in quality of the rail caused by welding, that is, hardening, fragility or softening at the weld heat affected zone. Therefore, it is desirable to produce rails having improved weldability.
High strength rails to date are roughly classified into two categories, that is
(1) as-rolled alloy steel rails and (2) heat treated carbon steel rails. One example of low alloyed heat treated rail is disclosed in Davies et al., U.S. Pat. No. 3,726,724. The rail steel disclosed in the above-mentioned patent is produced by adding to ordinary carbon steel for rails at least one hardening element such as Mn, Si, Cr, Ni and Mo in a total amount not more than 5%, at least one grain refining element such as Al, V, Nb, Ti and Zr and N in a stoichiometric proportion with respect to the amount of said grain refining ingredient already added. Subsequently said steel is formed into a rail by rolling, is normalized by reheating at above the A.sub.3 transformation point and is air cooled in the next stage, or is subjected to finish rolling under controlled rolling conditions at a temperature ranging from 700.degree. C. to 900.degree. C. The resulting rail has a ferrite and pearlite structure shown in FIG. 2 containing fine ferrite grains which are finer than A.S.T.M. grain size No. 8.
The rail steel thus produced has a ferrite and pearlite structure as mentioned above. The tensile strength thereof is about 70 to 81 kg/mm.sup.2 as disclosed in the specification, and is far less than the tensile strength of 120 kg/mm.sup.2 or more which value is the objective of the present invention.
The aforesaid "normalizing" of steel involves a final heat treatment at the austenite temperature followed by cooling with air, where there is the possibility that when said rail steel is subjected to welding, the quality of the rail may be deteriorated in the weld heat affected zone as compared with the base material zone.
Moreover, conventional as-rolled alloy steel rails stronger than the foregoing rail steel of U.S. Pat. No. 3,726,724 can be obtained by other processes, e.g. adding alloy elements, such as Si, Mn, Cr, Mo, V and the like to an ordinary carbon steel rail, then hot rolling said steel into a rail followed by ambient cooling to induce pearlite transformation as shown in FIG. 3. Such steel rail has a tensile strength of 100 to 120 kg/mm.sup.2, therefore, the foregoing process must employ a rather large quantity of alloy elements because of the fact that a high tensile strength is obtained at a rather slow cooling rate in the course of air cooling to form an austenite structure steel after rolling.
When the aforementioned rails are welded to each other to form an elongated rail, the cooling rate of the weld joint portion and the weld heat affected zone is somewhat faster than the cooling rate after hot rolling. Thereby martensite is grown locally which causes considerable hardening and embrittlement as compared with the base material rail, and is accompanied by serious problems in actual use such as, for instance, failure and uneven abrasion of the rail at the weld portion and track deterioration.
Further, it has been proposed to perform slow cooling after the welding operation, or post heat treatment at the welded portion and the like in order to prevent or diminish the above-mentioned growth of martensite, however, said treatment significantly decreases the welding efficiency and thus is hardly practical.
Moreover, two other processes have been used in many cases for rail welding and these involve the flash-butt welding process and the gas pressure welding process, in which no fusion welding is performed, so that the alloy elements are apt to be oxidized on the weld line. Sufficient joint strength could not be obtained thereby, resulting in problems such as breakage during transportation etc.
Next will be considered heat treated or head hardened rails. A convention example of said rail is disclosed in Fernand J. Dewez, Jr., et al, U.S. Pat. No. 3,124,492 entitled "METHOD FOR HEAT-TREATING RAILS." According to said patent, the head portion of the rail is heated at an austenite transformation temperature up to the maximum depth of 1.5" (38.1 mm), preferably to the depth between 1/4" and 1" (6.4 and 25.4 mm), and then the head portion of said rail thus heated is forcibly cooled by air blasting to effect pearlite transformation. Subsequently, the heat retained in the inner part of the rail is conducted to the surface thereof, thereby raising the surface temperature of the rail to 1250.degree. F. (677.degree. C.) for effecting self tempering, and in the next step, the rail is water cooled to ambient temperature. Thereby, the head portion of the rail is hardened on the surface layer thereof and exhibits tensile strength, with the aid of the foregoing tempering process, which is the so-called slack quenching-tempering, and the tensile strength thus obtained is more than about 120 kg/mm.sup.2.
Said process seeks to provide a pearlite structure in the heat treated region as shown in FIG. 4. When the rails produced by such slack quenching method are welded, they are subjected to cooling conditions which are remarkably different from their heat treatment. In particular, the cooling rate after welding is considerably slower than the cooling rate during the heat treatment; thereby it is impossible to avoid softening at the welded portion. Thus, the local deformation or abrasion of rails at the softened portion as well as the deterioration in the railroad bed originating from the above-mentioned defects are serious problems.
Further, Heller, U.S. Pat. No. 4,082,577 discloses a heat treatment to produce a pearlite structure by the following steps: heating to an austenite state, and commencing quenching from a temperature between 800.degree. to 850.degree. C. down to 100.degree. C. with boiling water. However, in this instance, notable deformation in rail, that is, bending of rail takes place, so that it is necessary to reform the configuration of rail after heat treatment.
Further, there is disclosed in Japanese Patent Publication Gazette Sho. 47(1972)-32, 168, column 2 lines 24 to 30, heat treatment of a rail for the purpose of increasing depth of the hardened layer at the head portion thereof in order to enhance the abrasion resistant property thereof. In such process, the head portion of said rail is heated to between 860.degree. and 1100.degree. C., then is cooled down slowly to 820.degree. to 850.degree. C. by ambient cooling, is subjected to a quenching treatment, subsequently is heated again up to the temperature of 400.degree. C. to 600.degree. C., and is subjected to tempering treatment. The thus treated portion of said rail can have a hardened layer, thereon with a thickness about twice as deep as that on conventionally heat treated rails. However, this process merely embodies a conventionally effected hardening and tempering treatment.
As a result of the foregoing conventional heat treatment, the microstructure thus obtained becomes tempered "sorbite," which is an obsolete term for tempered martensite, as disclosed in the METALS HANDBOOK (ASM), 8th Edition, vol. 1, "Properties and Selection of Metals," published in 1961 by AMERICAN SOCIETY FOR METALS, Chapter; DEFINITION RELATING TO METALS AND METALWORKING, Sorbite page 35; Ar.sub.1 ; page 39. Further, in said process, rail head portion is heated to 850.degree.-1100.degree. C. for coarsening the crystal grain of austenite structure, however, such procedure as set forth above is neither an attempt at controlling the growth of austenized grains nor at minimizing the pearlite block size. Therefore, when the rail thus quenched and tempered is subjected to welding, it inevitably causes deterioration in properties such as hardness of the above rail at the weld heat affected zone thereof.
As explained hereinbefore, existing high strength rails are certainly provided with base materials (that is, the nonwelded portion) having high strength characteristics and excellent properties but are thoroughly lacking in the ability to prevent the welded portion of rails from deteriorating in strength as well as structure and the like. Therefore, the welded portions of high strength rails tend to exhibit problematic conditions such as hardening, embrittlement or softening with respect to the base material portion.
In sum, many serious welding problems have been encountered in the above-metioned methods for producing high strength rails, i.e. in as-rolled alloy steel rails where alloy elements are employed on the basis of the cooling rates under ambient conditions after hot rolling and in heat treated carbon steel rails where the cooling rate in the heat treatment step is selected on the basis of the ingredients contained in the existing high carbon steels.